In general, semiconductor wafers are prepared in several steps, including (1) growing a single crystal ingot out of molten silicon, (2) sawing the single crystal ingot into wafers, (3) shaping or lapping the wafers, (4) performing a rough polish, and (5) depositing an epi layer of silicon substrate. The epi layer is often deposited using chemical vapor, high temperature deposition to form a single crystal silicon layer on the surface of the wafer. Once the wafers have been prepared, they are provided to a fabrication facility (fab) for further processing.
As fabs are processing smaller and smaller line widths and devices are continually shrinking, the wafer surface effects the entire fab processing. Furthermore, a particle that used to be "invisible" can now completely ruin a device. Therefore, the step of polishing becomes extremely important.
Conventional polishing includes placing the wafer on a chuck, such as a vacuum chuck that holds the wafer in place, and spraying the surface of the wafer with deionized water. Either the wafer or the outlet for deionized water is rotated to move the particles from the center of the wafer towards the outside of the wafer. Combinations of high pressure spray, a fast spinning wafer chuck and a brush placed in very close proximity to the wafer are often used. The high pressure spray effectively shoots the particles out of the wafer and the fast spinning chuck uses centrifugal force to remove the particles. The brush is a sponge-like piece for forcing a thin layer of water between it and the wafer to create pressure waves in the water.
The spray and spinning chuck methods are inefficient in removing particles, especially smaller particles. The brush method works well with the small particles, but becomes contaminated with and traps the larger particles. To effectively use the brush method, each brush must be routinely replaced. However, new brushes incur a break in period (several days) during which their cleaning quality is not optimized